HIMALAYAN “S-TYPE” GRANITE GENERATED FROM I-TYPE SOURCES

Although the origins of many granites remain intensely debated, partial melting of metasediments is generally accepted as the source of high-aluminum (peraluminous) Himalayan leucogranites – they are commonly considered pure “S-type” (sediment-sourced) granites. Here, geochronology and geochemistry show that partial melting of metaigneous rocks in the eastern Himalaya produced typical ~20 Ma peraluminous leucogranites – these supposed “S-type” leucogranites have I-type (igneous) sources. Inherited zircon from leucogranites and primary magmatic zircon from metaigneous rocks have indistiguishable 480 Ma ages, trace element compositions, and Hf isotope compositions, distinct from sedimentary sources. The single age for zircon in the orthogneisses and absence of characteristic detrital ~1000, ~1800, and ~2500 Ma zircon suggest the orthogneisses do not derive from prior partial melting of sediments – they are not themselves S-type granites. Experimental studies show that partial melting of metagranitic rocks predicts alkali major element chemistry of the studied leucogranites better than partial melting of metapelites, whereas other major element compositions do not distinguish source. Although high δ¹⁸O, high ⁸⁷Sr/⁸⁶Sr, and low ε_Nd for Himalayan leucogranites have been used to argue for a metasedimentary source, overlapping values occur in orthogneisses. Trace element compositions of these I-type leucogranites also fall within the range of other 8–30 Ma Himalayan leucogranites, but with distinct chemical characteristics, especially including depletions in rare metals. Although sediment-sourced leucogranites also occur and may dominate some areas, igneous-sourced leucogranites are likely common in large hot orogens. Past researchers have asserted that I-type granites in collisions represent juvenile additions to the crust, but in most collisional orogens they more likely represent crustal reworking.